1. Field of the Invention
The present invention relates to a DC stack type AC-coupling BC diode attenuator, and more particularly to an attenuator adapted to have substantially constant distortion characteristics up to a certain (high) transmission power level both in the attenuating and non-attenuating states.
2. Background Art
In recent years, GaAs-HBT (Heterojunction Bipolar Transistor) power amplifiers have been widely used as cellular phone power amplifiers for CDMA (Code Division Multiple Access), etc. and wireless LAN power amplifiers.
GaAs-HBTs do not require a negative gate bias voltage, which allows them to operate on a single power source. Furthermore, their device characteristics do not vary as much as those of GaAs-FETs. Therefore, recently, GaAs-HBTs have been increasingly used in GaAs power amplifiers for cellular phones and wireless LANs.
However, a GaAs-HBT process usually cannot form a RF (radio frequency) switching device whose channel can be turned on by application of a gate voltage alone. Therefore, such a switching device is made up of a base-collector junction diode (or BC diode), which has characteristics similar to those of a p-i-n junction (or diode). (See, e.g., Japanese Laid-Open Patent Publication No. 2003-347870)
FIG. 14 is a circuit diagram of a conventional switch employing a BC diode. Specifically, this switch includes: a diode D1 having an anode and a cathode connected to an input terminal IN and an output terminal OUT, respectively; a control voltage terminal Vc1 connected to the anode of the diode D1 through an RF blocking inductor L1; and a resistance R1 and an RF blocking inductor L2 connected in series between the cathode of the diode D1 and a ground point.
In the switch shown in FIG. 14, when a voltage higher than the turn-on voltage of the diode D1 (that is, approximately 1.25 V) is applied to the control voltage terminal Vc1, the diode D1 switches from an off state to an on state, causing a current Idc determined by the resistance R1 to flow through the diode D1. Since the diode D1 thus assumes the on state, the RF signal input to the input terminal IN is passed to the output terminal OUT. On the other hand, when a voltage lower than the turn-on voltage of the diode D1 (including a negative bias) is applied to the control voltage terminal Vc1, the diode D1 assumes an off state, preventing the RF signal from passing through.
FIG. 15 is a circuit diagram of a conventional attenuator employing a BC diode. In addition to the components shown in FIG. 14, the attenuator includes: a resistance R01 connected at one end to the anode of the diode D1; a resistance R02 connected at one end to the cathode of the diode D1; a diode D2 having an anode and a cathode, the anode being grounded through a capacitance C2, the cathode being connected to the other end of the resistance R02 and connected to the other end of the resistance R01 through a capacitance C1; and a control voltage terminal Vc2 connected to the anode of the diode D2 through the RF blocking inductor L2 and a resistance R4.
In the attenuator shown in FIG. 15, when a voltage higher than the turn-on voltage of the diode D1 is applied to the control voltage terminal Vc1 at the same time that a voltage lower than the turn-on voltage of the diode D2 (including a negative bias) is applied to the control voltage terminal Vc2, the attenuator assumes a non-attenuating state and hence the RF signal input to the input terminal IN is passed to the output terminal OUT with no attenuation. On the other hand, when a voltage lower than the turn-on voltage of the diode D1 is applied to the control voltage terminal Vc1 at the same time that a voltage higher than the turn-on voltage of the diode D2 is applied to the control voltage terminal Vc2, the attenuator assumes an attenuating state determined by the resistances R01 and R02 and the on-resistance of the diode D2. It should be noted, however, that if the capacitances C1 and C2 are formed on the GaAs chip and hence are small, the impedance values of these capacitances at the operational frequencies are also factors in determining the amount of attenuation.
FIG. 16 is a diagram illustrating the RF signal input to the input terminal IN. In the figure, I(t) represents the RF signal, that is, the current flowing through the diode D1; Imax represents the maximum value of the amplitude of the current; and T represents the period. The maximum allowable input power to the attenuator is set such that the insertion loss is acceptably low. Specifically, the current I(t) flowing through the diode D1 is limited such that the time integral of the current for a half cycle (equal to the total amount of charge flowing during that cycle) is smaller than the product of the bias current Idc and a time constant t, as represented by equation (1) below. It should be noted that the time constant t is determined by the bonding material and the bonding conditions of the diode (i.e., the impurity concentration and thickness of the I-layer, or high resistance layer, etc.).
                    [                              Equation                    ⁢                                          ⁢          1                ]                                                                                  ∫            0                          T              /              2                                ⁢                                    I              ⁡                              (                t                )                                      ⁢                          ⅆ              t                                      =                              I            ⁢                                                  ⁢                          max              /                              (                                  π                  *                  f                                )                                              <                      Idc            *            τ                                              (        1        )            
Thus, the lower the frequency, the smaller the power delivered through the attenuator (assuming the same bias current Idc). Especially, since the BC layer of a BC diode formed by a GaAs-HBT process is determined by the RF characteristics of the HBT, there is no freedom in design of the structure of the BC layer. Furthermore, the time constant t of a GaAs p-i-n diode is roughly two orders of magnitude smaller than that of an Si p-i-n diode, resulting in significantly reduced maximum allowable input power. Therefore, a switch or an attenuator employing a BC diode requires a large bias current to achieve desired maximum allowable transmission power.
To solve the above problems, the inventor has devised the switch and the attenuator shown in FIGS. 17 and 18, respectively.
The switch shown in FIG. 17 includes: a diode D1 having an anode and a cathode that are connected to an input terminal IN and an output terminal OUT, respectively; a control voltage terminal Vc1 connected to the anode of the diode D1 through an RF blocking inductor L1; a diode D2 having an anode connected to the cathode of the diode D1 and a cathode connected to the anode of the diode D1 through a capacitance C1; and a resistance R1 and an RF blocking inductor L2 connected in series between the cathode of the diode D2 and a ground point.
The attenuator shown in FIG. 18 is configured such that: the anode and cathode of a diode D1 are connected to an input terminal IN and an output terminal OUT, respectively; a control voltage terminal Vc1 is connected to the anode of the diode D1 through an RF blocking inductor L1; and the anode of a diode D2 is connected to the cathode of the diode D1, and the cathode of the diode D2 is connected to the anode of the diode D1 through a capacitance C1.
Further, one end of a resistance R01 is connected to the cathode of the diode D2, and one end of a resistance R02 is connected to the cathode of the diode D1 through a capacitance C2; the anode of a diode D3 is connected to the other ends of the resistances R01 and R02 through a capacitance C3, and the cathode of a diode D4 is connected to the other ends of the resistances R01 and R02 through a capacitance C4; and one end of a capacitance C5 is connected to the cathode of the diode D3 and to the anode of the diode D4, and the other end of the capacitance C5 is grounded.
Further, a control voltage terminal Vc2 is connected to the cathode of the diode D2 through an RF blocking inductor L2 and a resistance R2; a control voltage terminal Vc3 is connected to the anode of the diode D3 through an RE blocking inductor L3 and a resistance R3; and a control voltage terminal Vc4 is connected to the cathode of the diode D4 through an RF blocking inductor L4 and a resistance R4.
In the attenuator shown in FIG. 18, when voltages higher than the turn-on voltages of the diodes D1 and D4 (that is, high voltage levels) are applied to the control voltage terminals Vc1 and Vc4, respectively, at the same time that voltages lower than the turn-on voltages of the diodes D2 and D3 (that is, low voltage levels) are applied to the control voltage terminals Vc2 and Vc3, respectively, the attenuator assumes a non-attenuating state and hence the RF signal input to the input terminal IN is passed to the output terminal OUT with no attenuation. On the other hand, when voltages lower than the turn-on voltages of the diodes D1 and D4 (that is low voltage levels) are applied to the control voltage terminals Vc1 and Vc4, respectively, at the same time that voltages higher than the turn-on voltages of the diodes D2 and D3 (that is, high voltage levels) are applied to the control voltage terminals Vc2 and Vc3, respectively, the attenuator assumes an attenuating state. It should be noted that the amount of attenuation is determined by the resistances R01 and R02, the values of the capacitances C1 to C6, the bias current Idc, and the bias voltage.
Further, the diodes D1 and D2 are DC connected in series, as well as being AC connected in parallel through the capacitance C1. This allows the DC bias current Idc to flow through both the diodes D1 and D2 when a high level voltage is applied to the control voltage terminal Vc1. When the attenuator circuit is viewed as an AC circuit (viewed in terms of AC), a DC current twice as large as the bias current Idc flows. This means that the value Imax in equation (1) is increased by a factor of approximately 2. Since the maximum allowable transmission power is Ro*(Imax)2/2 (where Ro is the characteristic impedance), the attenuator shown in FIG. 18 can deliver transmission power approximately 4 times as large as the maximum allowable transmission power of the attenuator shown in FIG. 15.
FIG. 19 shows the power transfer characteristics of the attenuators shown in FIGS. 15 and 18. As shown in FIG. 19, the maximum allowable transmission power level of the attenuator in FIG. 18 is approximately 6-8 dB higher than that of the attenuator in FIG. 15 (assuming the same bias current).
FIG. 20 depicts graphs illustrating the output characteristics of the attenuator of FIG. 18 when it is in its attenuating and non-attenuating states. (These graphs were obtained experimentally.) It should be noted that the vertical axis represents signal distortion, namely, the third order intermodulation distortion Pim3 (two input signals). As can be from the figure, the attenuator exhibited a dramatic increase in signal distortion when it provided 20 dB attenuation at high input power. This dramatic increase in signal distortion was not observed in the non-attenuating state. When the attenuator is used in a system using a modulating signal, such an increase in signal distortion leads to degradation of the signal quality of the system, which is not desirable.